HETEROGENEOUS <EM>HES1</EM> EXPRESSING NEURAL STEM CELLS OF EMBRYONIC CORTEX VARY IN LINEAGE COMMITMENT

Neural stem cells (NSCs) of the cortex are heterogeneous in Hes1 gene activation. This differential mode of Hes1 activation gives rise to two NSC subtypes: Notch-independent Hes1-expressing (NIHes1) NSCs and Notch-dependent Hes1-expressing (NDHes1) NSCs. Radial glial cells (RGCs), which are well studied, belong to the NDHes1 subtype and are maintained through Notch-mediated Hes1 activation. This study characterizes the second subtype, NIHes1 NSCs, which maintain Hes1 expression without Notch signaling and therefore do not require cell-cell interaction.
Using single-cell transcriptomics (scRNA-seq), we identified NIHes1 NSCs and characterized their functional gene signatures. Our analyses confirm that NIHes1 NSCs maintain active Hes1 expression without Notch/CBF1 interaction. These NSCs also exhibit active Wnt signaling. Further, the two NSC subtypes show distinct functional signatures associated with different differentiated fates. NDHes1 NSCs showed upregulation of astrocytic genes (Mlc1, Acsbg1, Aldh1l1), whereas Neurog2, a major proneural gene, is more highly expressed in NIHes1 NSCs. Pseudotime analysis indicates that NIHes1 NSCs act as ancestral precursor NSCs that generate RGCs and intermediate progenitor cells (IPCs). We also used a Nestin-CreERT2;NIHes1^fl/fl conditional knockout mouse model to selectively remove the Notch-independent promoter region of the Hes1 gene in Nestin-positive NSCs. Loss of NIHes1 expression significantly alters the NSC niche, increasing RGCs and reducing IPCs.
Altogether, the Hes1⁺ NSCs are more heterogeneous than previously recognized. In contrast to earlier understanding, IPCs could be originating from a specific subclass of Hes1-expressing NSCs, the NIHes1 NSCs and not from NDHes1 RGCs. This study highlights the role of NIHes1 NSCs in establishing the NSC niche.